In a system using such single-sideband (SSB) or vestigial-sideband (VSB) modulation, only a coherent demodulation at the receiving end will prevent signal distortions due to phase errors. Such coherent demodulation, however, requires the transmission of a pilot tone -- harmonically related to carrier frequency -- outside the signal band. The pilot tone, aside from being itself subject to so-called phase jitters during transmission, requires the expenditure of additional energy and also occupies a portion of the available frequency range which would otherwise be available to neighboring signal bands. These phase jitters, particularly annoying in telephone channels, dictate a broadening of the band allotted to the pilot tone which is contrary to the desiderata of low noise interference and limited frequency spread.
Various systems have therefore been devised which dispense with the pilot tone and recover the original base-band signal by phase or amplitude demodulation from an asymmetrically modulated carrier. Foremost among these techniques is the use of a locally generated oscillation whose frequency approximates that of the carrier and which is heterodyned with the incoming wave to yield the base-band signal with as little distortion as possible. Reference may be made in this connection to U.S. Pat. No. 3,634,773 in the name of Hisashi Kobayashi and to an article by that inventor entitled "Simultaneous Adaptive Estimation and Decision Algorithm for Carrier-Modulated Data-Transmission Systems", IEEE Transactions on Communication Vol. COM-19, No. 3, pages 268 - 280, June 1971. The Kobayashi system employs adjustable equalizers of the so-called "transverse-filter" type.